1. Field of the Invention
The present invention relates to a process for the preparation of bulk or thin-film single-crystals of cubic sesquioxides (space group No. 206, Ia-3) of scandium, yttrium or rare earth metals doped with lanthanide ions having a valency of +III by a high-temperature flux growth technique and to the various applications of the single-crystals obtained according to this process, in particular in the optical field.
2. Description of Related Art
A laser is a device which emits light (electromagnetic radiation) amplified by stimulated emission. The term laser is an acronym originating from “light amplification by stimulated emission of radiation”. The laser produces a spatially and temporally coherent light based on the laser effect. By virtue of the stimulated emission process, the photon emitted has the same wave vector, the same polarization and the same phase as the photon moving through the optical cavity. This results in a source of coherent radiation.
Various types of laser exist, among which may in particular be mentioned gas lasers, chemical lasers, organic dye lasers, metal vapor lasers, solid-state lasers and semiconductor lasers.
Solid-state lasers use solid media, such as crystals or glasses, as medium for the emission (spontaneous and stimulated) of photons and amplifier medium. The amplifier medium, or also gain medium, is composed of an optically active material comprising a matrix (glass or crystal) rendered optically active by doping with an ion which absorbs the radiation from an optical pumping source and which is de-excited by emission of photons. The first laser is a ruby laser, the emission of which originates from the Cr3+ ion. Other ions are much used: the majority are rare earth metal ions: Nd3+, Yb3+, Pr3+, Er3+, Tm3+, . . . , or also transition metal ions, such as Ti3+ or Cr3+, inter alia. The emission wavelength of the laser depends essentially on the doping ion for the rare earth metal ions and on the properties of the matrix in all cases, the influence of the latter being much greater in the case of the transition metal ions. Thus, glass doped with neodymium does not emit at the same wavelength (1053 nm) as the crystalline solid known as yttrium-aluminum-garnet (YAG) and composed of Y3Al5O12 doped with neodymium (1064 nm). Solid-state lasers operate in continuous mode or in pulsed mode (pulses from a few microseconds to a few femtoseconds). They are capable of emitting equally well in the visible region, the near infrared region, the middle infrared region and the ultraviolet region.
Above a crystal dimension of acceptable optical quality, these lasers make it possible to obtain powers of the order of approximately ten watts continuously and higher powers in pulsed mode. They are used for both scientific and industrial applications, such as welding, marking and cutting of materials.
In addition to their use in the manufacture of high-power lasers and/or short-pulse lasers, these solid materials, formed of a matrix and of a doping ion, can also be used in the manufacture of eye-safety lasers, of lasers for surgery and/or ophthalmology (diode-pumped lasers, pulsed or continuous, in the red region, the green region and up to the middle infrared region), of scintillators, of waveguides, of bolometers (detectors having heat/light discrimination), for optical cooling, as luminophoric materials or alternatively as materials for the storage and handling of quantum information.
At the current time, the most promising crystalline solids for all of these applications, and in particular for the manufacture of lasers, are cubic (thus isotropic) sesquioxides of formula R2O3 in which R represents one or more elements chosen from metals having a valency of III, such as scandium, yttrium and the rare earth metals (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), doped with rare earth metal ions. Some of them exhibit in particular a greater thermal conductivity than that of YAG doped with rare earth metal ion, which is nevertheless the most widely used laser material at the current time. These cubic sesquioxides are also advantageous insofar as they can be easily doped with rare earth metal ions and have a high density (of the order of 4 to 9.5 g.cm−3 approximately). Furthermore, yttrium, scandium, gadolinium and lutetium sesquioxides exhibit low phonon energies in comparison with the majority of oxides, in particular YAG.
At the current time, these materials are mainly obtained in the form of transparent ceramics prepared by high pressure and high temperature sintering, preferably under vacuum. However, these ceramics exhibit a polycrystalline microstructure with numerous grain boundaries, diminishing the physical properties at the basis of their applications (diffusion of photons, presence of impurities, low thermal conductivity, and the like).
In point of fact, in order to be able to be used in an optimum way in these various applications (laser, scintillator, and the like), these cubic sesquioxides have to be provided in the form of single-crystals having a size in the millimeter to centimeter range and having a satisfactory optical quality.
As a result of their very high melting point (T>2400° C.) and of their high chemical reactivity in the molten state, conditions for producing these cubic sesquioxides in the form of single-crystals and with a satisfactory optical quality prove to be difficult to implement, expensive and sometimes dangerous and most of the time require postgrowth treatments for several hours at high temperature (annealing).
Various techniques for the synthesis of single-crystals of rare earth metal sesquioxides have been reviewed by Akira Yoshikawa and Andrey Novoselov (book: Shaped Crystals; publisher: Springer Berlin Heidelberg; Part III, pages 187-202; Copyright 2007). These authors remind the reader that, as a result of their very high chemical reactivity in the molten state, these materials in principle have to be synthesized according to “crucibleless” techniques, such as, for example, the Verneuil method (R. A. Lefever et al., Rev. Sci. Instrum., 1963, 33, 769), the zone melting technique known under the term “Laser-Heated Pedestal Growth” (LHPG), as described by D. B. Gasson et al., J. Mater. Sci., 1970, 5, 100, or alternatively according to the cold-crucible or self-crucible technique, also known under the term “skull-melting” and described in particular by V. V. Osiko, J. Cryst. Growth, 1983, 65, 235. Other crystalline growth techniques, such as that of J. Czochralski (Z. Phys. Chem., 1918, 92, 219), carry out the crystallogenesis in crucibles. It is then essential to carry out the growth of the crystals in special crucibles formed of a material having a higher melting point than the melting point of the material to be grown, that is to say greater, in the present case, than 2400° C., such as crucibles made of rhenium, the melting point of which exceeds 3100° C. (L. Fornasiero et al., Cryst. Res. Technol., 1999, 34, 255). However, these techniques exhibit the disadvantage of resulting in single-crystals which are too small (at most of the order of a few millimeters) and/or which exhibit an optical quality insufficient for the applications envisaged. Akira Yoshikawa and Andrey Novoselov for their part provide for the use of a micro-pulling down (μ-PD) technique for carrying out the synthesis of single-crystals of rare earth metal sesquioxides, the principle and the use of which make it possible to produce single-crystal fibers. It concerns drawing fiber downward from a micrometer-size nozzle placed under a crucible in which the powder mixture is brought to its melting point. However, in the case of the growth of materials of the R2O3 type as defined above, this technique, however, requires a special and expensive device (sometimes not able to be operated industrially) and makes it possible to obtain only crystals in the form of fibers. It thus does not make it possible to access large-sized bulk single-crystals. In contrast, the technique known under the acronym HEM for Heat Exchanger Method, provided by V. Peters et al. (J. Cryst. Growth, 2002, 237-239, 879-883), makes it possible to obtain monocrystals of the R2O3 type having a size of several centimeters by the slow cooling of the bottom of a crucible made of rhenium comprising a molten bath of R2O3 brought to the melting point (T>2400° C.). Nevertheless, this technique is impossible to transfer to the industrial scale as a result of its very high cost (price of rhenium, for example), of its dangerousness and of the high chemical reactivity with one another of the elements present in the furnace. This is because the refractory ceramics and the rhenium crucible, in the presence of hydrogen H2 (explosive gas of the growth atmosphere and consequently highly dangerous at high temperature), exhibit strong signs of chemical attack (darkening of the refractory ceramics, dissolution of the crucible resulting in contamination of the crystals) after growth and then require their replacement virtually systematically after only a few growths.
Provision has also already been made, in particular in patent application US 2009/0151621, to synthesize sesquioxides of R2O3 type, in particular of scandium, in the single-crystal form by the hydrothermal route. This process consists in dissolving an R2O3 powder, where R represents one or more elements chosen from scandium, yttrium and lanthanides, in an aqueous solution comprising hydroxide ions and in then bringing the resulting solution to a temperature of 450° C.; to 750° C. under a pressure which can range from 4 to 40 kpsi (i.e., approximately from 3×107 to 3×109 Pa.). This process results in R2O3 single-crystals, the size of which can vary from the order of a few mm to a centimeter; however, it requires operating at a very high pressure and consequently requires a suitable and necessarily expensive device. This technique exhibits the disadvantage of using solutions which are highly corrosive with their environment, then resulting in chemical attack on the silver crucibles used as container. This then results in the contamination of the crystals by the Ag+ ions, which is additional to that by the OH− ions initially present in the solution. Furthermore, the rate of growth of the crystals by the hydrothermal route is very slow (generally less than 0.5 mm/day), which requires extremely lengthy growth times to hope to achieve a crystal size of the order of a centimeter.